Ghislain & Marie David de Lossy via Getty Images
ワシントンD.C.に住む9歳のルイサ・マリア・バスケス・クルスちゃんは2013年、かかりつけの小児科医から、一風変わった処方箋を受け取った。そこには「外でもっと長い時間遊ぶように」と書かれていたのだ。
彼女を診断したのは、「ユニティ・ヘルスケア」（低所得層や保険に加入していない家庭に医療サービスを提供している）の小児科医、ロバート・ザール医師だ。同医師によれば、彼女の治療プログラムは「大成功」を収めたそうだ。彼女の肥満問題は克服され、持病のぜんそくも十分にコントロールできるようになったという。
ザール医師は、公園での外遊びを推奨するこのプログラムを2013年7月に始めて以来、自然の中で時間を過ごすよう指示する処方箋を600枚以上書いている。肥満から来る糖尿病などの病気のほか、うつ病や注意欠陥・多動性障害（ADHD）などの精神的な問題も含めて、さまざまな健康問題を予防・治療するためだ。
一部の研究者によれば、屋外で時間を過ごさせることは、ADHDの症状を、メチルフェニデートを含む薬を飲んだ場合と同じ程度にまで和らげるという。特に、公園など自然環境の中での散歩は、町なかや家の近所を歩くよりもはるかに高い効果が得られるという。また、自然の中で時間を過ごすことは、視力や骨密度の向上につながるという報告もある。
つまり、自然の中で過ごすことは健康に良い。しかし、ある要因がこの「療法」の障害になりつつある。温暖化に伴う害虫の増加だ。
ワシントンD.C.は、気温の上昇などのために蚊の増加に見舞われているアメリカ国内の地域のひとつだ。特に増えているヒトスジシマカ（ヤブカ）は、デング熱や西ナイル熱、そして近年流行しているチクングニア熱など30種類を超える感染症を媒介する生物として知られている。
さらに、気候変動に伴い、ダニたちも北上していると報道もされている。このやっかいな生き物は、ライム病やツツガムシ病などの感染症を引き起こすことがあり、森や、低木が多い場所や、草で覆われた場所に生息していることが多い。町中の公園にも生息しているようだ。
ほかにも、毒をもつファイアーアント（Fire Ant。カミアリ／ヒアリ）や、攻撃的なクレイジーアント（日本語版記事）、あるいはカメムシも増加していると報告されているが、温暖化が影響するのは昆虫だけではない。アレルギーや喘息の原因となる花粉の数も増えているという。また、かぶれの原因となるポイズン・アイビー（ウルシ科の一種）も急増し、その毒性が高くなっていると報道されている。ポイズン・アイビーは、アラスカ州とハワイ州を除く米本土48州では、カリフォルニア州以外のすべての州で勢力を拡大している。
ザール医師は、虫除けスプレーなどの手段を推奨することによって「外遊びという治療」を勧め続けるというが、温暖化は、アウトドアで過ごしたい人たちにとってもやっかいな問題になりつつあるようだ。
[Lynne Peeples(English) 日本語版：佐藤卓、合原弘子/ガリレオ]
デング熱 日本で感染拡大
デング熱／新宿御苑が閉鎖(01 of34)
デング熱の感染拡大で閉鎖となった新宿御苑＝７日午後、東京都新宿区\n\n撮影日：2014年09月07日 (credit:時事通信社)
デング熱／来場者に虫よけスプレー(02 of34)
イベントの来場者に虫よけスプレーをするスタッフ＝６日午後、東京都渋谷区の代々木公園\n\n撮影日：2014年09月06日 (credit:時事通信社)
デング熱／殺虫剤を散布する作業員 (03 of34)
デング熱の感染場所の疑いがある新宿中央公園で、殺虫剤を散布する作業員＝2014年09月05日夜、東京都新宿区 (credit:時事通信社)
デング熱／殺虫剤を散布する作業員 (04 of34)
採集した蚊からデング熱ウイルスが検出されたことを受け、殺虫剤を散布する作業員＝2014年09月05日午後、東京・代々木公園 (credit:時事通信社)
デング熱／デング熱対策で排水作業 (05 of34)
デング熱対策で、蚊の発生を抑えるため池の排水作業をする東京都公園協会の職員ら＝2014年09月01日午後、東京・代々木公園 (credit:時事通信社)
Japan Copes With First Dengue Fever Case In Nearly 70 Years(06 of34)
TOKYO, JAPAN - AUGUST 28: A workman sprays pesticide in Yoyogi Park on August 28, 2014 in Tokyo, Japan. Sections of Yoyogi Park were closed to the public today as they underwent fumigation after three people were diagnosed with dengue fever after suffering mosquitoe bites in the park. According to a Health Ministry statement, these were the first domestic cases of dengue fever in Japan in 70 years. (Photo by Chris McGrath/Getty Images) (credit:Chris McGrath via Getty Images)
Japan Copes With First Dengue Fever Case In Nearly 70 Years(07 of34)
TOKYO, JAPAN - AUGUST 28: A workman sprays pesticide in Yoyogi Park on August 28, 2014 in Tokyo, Japan. Sections of Yoyogi Park were closed to the public today as they underwent fumigation after three people were diagnosed with dengue fever after suffering mosquitoe bites in the park. According to a Health Ministry statement, these were the first domestic cases of dengue fever in Japan in 70 years. (Photo by Chris McGrath/Getty Images) (credit:Chris McGrath via Getty Images)
Japan Copes With First Dengue Fever Case In Nearly 70 Years(08 of34)
TOKYO, JAPAN - AUGUST 28: Workers spray pesticide in Yoyogi Park on August 28, 2014 in Tokyo, Japan. Sections of Yoyogi Park were closed to the public today as they underwent fumigation after three people were diagnosed with dengue fever after suffering mosquitoe bites in the park. According to a Health Ministry statement, these were the first domestic cases of dengue fever in Japan in 70 years. (Photo by Chris McGrath/Getty Images) (credit:Chris McGrath via Getty Images)
Japan Copes With First Dengue Fever Case In Nearly 70 Years(09 of34)
TOKYO, JAPAN - AUGUST 28: A workman sprays pesticide in Yoyogi Park on August 28, 2014 in Tokyo, Japan. Sections of Yoyogi Park were closed to the public today as they underwent fumigation after three people were diagnosed with dengue fever after suffering mosquitoe bites in the park. According to a Health Ministry statement, these were the first domestic cases of dengue fever in Japan in 70 years. (Photo by Chris McGrath/Getty Images) (credit:Chris McGrath via Getty Images)
Japan Copes With First Dengue Fever Case In Nearly 70 Years(10 of34)
TOKYO, JAPAN - AUGUST 28: A workman sprays pesticide in Yoyogi Park on August 28, 2014 in Tokyo, Japan. Sections of Yoyogi Park were closed to the public today as they underwent fumigation after three people were diagnosed with dengue fever after suffering mosquitoe bites in the park. According to a Health Ministry statement, these were the first domestic cases of dengue fever in Japan in 70 years. (Photo by Chris McGrath/Getty Images) (credit:Chris McGrath via Getty Images)
Japan Copes With First Dengue Fever Case In Nearly 70 Years(11 of34)
TOKYO, JAPAN - AUGUST 28: A workman sprays pesticide in Yoyogi Park on August 28, 2014 in Tokyo, Japan. Sections of Yoyogi Park were closed to the public today as they underwent fumigation after three people were diagnosed with dengue fever after suffering mosquitoe bites in the park. According to a Health Ministry statement, these were the first domestic cases of dengue fever in Japan in 70 years. (Photo by Chris McGrath/Getty Images) (credit:Chris McGrath via Getty Images)
Japan Copes With First Dengue Fever Case In Nearly 70 Years(12 of34)
TOKYO, JAPAN - AUGUST 28: A workman sprays pesticide in Yoyogi Park on August 28, 2014 in Tokyo, Japan. Sections of Yoyogi Park were closed to the public today as they underwent fumigation after three people were diagnosed with dengue fever after suffering mosquitoe bites in the park. According to a Health Ministry statement, these were the first domestic cases of dengue fever in Japan in 70 years. (Photo by Chris McGrath/Getty Images) (credit:Chris McGrath via Getty Images)
Japan Copes With First Dengue Fever Case In Nearly 70 Years(13 of34)
TOKYO, JAPAN - AUGUST 28: Workers spray pesticide in Yoyogi Park on August 28, 2014 in Tokyo, Japan. Sections of Yoyogi Park were closed to the public today as they underwent fumigation after three people were diagnosed with dengue fever after suffering mosquitoe bites in the park. According to a Health Ministry statement, these were the first domestic cases of dengue fever in Japan in 70 years. (Photo by Chris McGrath/Getty Images) (credit:Chris McGrath via Getty Images)
Japan Copes With First Dengue Fever Case In Nearly 70 Years(14 of34)
TOKYO, JAPAN - AUGUST 28: A workman sprays pesticide in Yoyogi Park on August 28, 2014 in Tokyo, Japan. Sections of Yoyogi Park were closed to the public today as they underwent fumigation after three people were diagnosed with dengue fever after suffering mosquitoe bites in the park. According to a Health Ministry statement, these were the first domestic cases of dengue fever in Japan in 70 years. (Photo by Chris McGrath/Getty Images) (credit:Chris McGrath via Getty Images)
Japan Copes With First Dengue Fever Case In Nearly 70 Years(15 of34)
TOKYO, JAPAN - AUGUST 28: A workman sprays pesticide in Yoyogi Park on August 28, 2014 in Tokyo, Japan. Sections of Yoyogi Park were closed to the public today as they underwent fumigation after three people were diagnosed with dengue fever after suffering mosquitoe bites in the park. According to a Health Ministry statement, these were the first domestic cases of dengue fever in Japan in 70 years. (Photo by Chris McGrath/Getty Images) (credit:Chris McGrath via Getty Images)
BRAZIL-SCIENCE-HEALTH-DENGUE-MOSQUITO(16 of34)
TO GO WITH AFP STORY by Natalia Ramos\nA researcher examines transgenic Aedes aegypti mosquitos kept in cages to collect their eggs, at a laboratory of biotech company Oxitec, in Campinas, 100 km from Sao Paulo, Brazil, on August 21, 2014. Oxitec produces genetically modified mosquitoes to fight dengue, a deadly tropical disease more prevalent in Brazil than anywhere else in the world. The factory developed a technology where eggs of Aedes aegypti receive microinjection of DNA with genes that produce a protein which prevents their offsprings from reaching adulthood and thus reducing the total population of transmitters of dengue. AFP PHOTO / NELSON ALMEIDA (Photo credit should read NELSON ALMEIDA/AFP/Getty Images) (credit:NELSON ALMEIDA via Getty Images)
Cuba fights the Aedes aegypti by fumigating frecuently in(17 of34)
SANTA CLARA, VILLA CLARA, CUBA - 2014/07/30: Cuba fights the Aedes aegypti by fumigating frecuently in homes and business, the country is suffering Dengue and Chikungunya virus. (Photo by Roberto Machado Noa/LightRocket via Getty Images) (credit:Roberto Machado Noa via Getty Images)
Cuba fights the Aedes aegypti by fumigating frecuently in(18 of34)
SANTA CLARA, VILLA CLARA, CUBA - 2014/07/30: Cuba fights the Aedes aegypti by fumigating frecuently in homes and business, the country is suffering Dengue and Chikungunya virus. (Photo by Roberto Machado Noa/LightRocket via Getty Images) (credit:Roberto Machado Noa via Getty Images)
HAITI-HEALTH-FUMIGATION(19 of34)
A worker from Haiti\'s Ministry of Public Health and Population sprays chemical to exterminate mosquitoes in a neighborhood of Petion Ville in Port-au-Prince on May 21, 2014. A worker said the procedure is to help prevent chikungunya, dengue, malaria and filariose. AFP PHOTO / Hector RETAMAL (Photo credit should read HECTOR RETAMAL/AFP/Getty Images) (credit:HECTOR RETAMAL via Getty Images)
HAITI-HEALTH-FUMIGATION(20 of34)
A worker from Haiti\'s Ministry of Public Health and Population sprays chemical to exterminate mosquitoes in a neighborhood of Petion Ville in Port-au-Prince on May 21, 2014. A worker said the procedure is to help prevent chikungunya, dengue, malaria and filariose. AFP PHOTO / Hector RETAMAL (Photo credit should read HECTOR RETAMAL/AFP/Getty Images) (credit:HECTOR RETAMAL via Getty Images)
Government Construction in Rio Favela Affects Children's Health(21 of34)
RIO DE JANEIRO, BRAZIL - MAY 10: Children play in the rubble of a former home next to construction in the Manguinhos \'favela\' on May 10, 2014 in Rio de Janiero, Brazil. Residents from one section of the favela, visited by Pope Francis in July 2013, have been affected by construction from an improvement works project begun in 2008. A full sanitation system has yet to be built. Some have been evicted from the area as Rio prepares for the 2014 FIFA World Cup and Rio 2016 Olympic Games. Children living in the affected area have been injured from playing in the rubble while complaining of breathing problems from dust and stomach aches. One child contracted Dengue fever twice and family members believe it was caused by stagnant water in the construction area. (Photo by Mario Tama/Getty Images) (credit:Mario Tama via Getty Images)
Government Construction in Rio Favela Affects Children's Health(22 of34)
RIO DE JANEIRO, BRAZIL - MAY 10: Children play in the rubble of a former home next to ongoing construction in the Manguinhos \'favela\' on May 10, 2014 in Rio de Janiero, Brazil. Residents from one section of the favela, visited by Pope Francis in July 2013, have been affected by construction from an improvement works project begun in 2008. A full sanitation system has yet to be built. Some have been evicted from the area as Rio prepares for the 2014 FIFA World Cup and Rio 2016 Olympic Games. Children living in the affected area have been injured from playing in the rubble while complaining of breathing problems from dust and stomach aches. One child contracted Dengue fever twice and family members believe it was caused by stagnant water in the construction area. (Photo by Mario Tama/Getty Images) (credit:Mario Tama via Getty Images)
INDONESIA-HEALTH-DENGUE(23 of34)
A worker fumigates an area to kill mosquitoes that may carry dengue fever in Jimbaran on the resort island of Bali on April 15, 2014. Dengue fever is spread by mosquitoes and causes joint pain, nausea, high fevers, and rashes and in severe cases, it results in death. AFP PHOTO/SONNY TUMBELAKA (Photo credit should read SONNY TUMBELAKA/AFP/Getty Images) (credit:SONNY TUMBELAKA via Getty Images)
INDONESIA-HEALTH-DENGUE(24 of34)
A worker fumigates an area to kill mosquitoes that may carry dengue fever in Jimbaran on the resort island of Bali on April 15, 2014. Dengue fever is spread by mosquitoes and causes joint pain, nausea, high fevers, and rashes and in severe cases, it results in death. AFP PHOTO/SONNY TUMBELAKA (Photo credit should read SONNY TUMBELAKA/AFP/Getty Images) (credit:SONNY TUMBELAKA via Getty Images)
PHILIPPINES-HEALTH-DENGUE(25 of34)
Employees from the Philippines Department of Health perform a \'mosquito dance\' with mascots (foreground) to create public awareness of vector-borne diseases in observance of World Health Day in Manila on April 7, 2014. According to the World Health Organization 40 percent of the world\'s population is at risk from dengue fever. Dengue is spread by mosquitoes that breed in stagnant water and usually bite people during daytime. AFP PHOTO / Jay DIRECTO (Photo credit should read JAY DIRECTO/AFP/Getty Images) (credit:JAY DIRECTO via Getty Images)
PHILIPPINES-HEALTH-DENGUE(26 of34)
Employees from the Philippines Department of Health perform a \'mosquito dance\' with mascots (foreground) to create public awareness of vector-borne diseases in observance of World Health Day in Manila on April 7, 2014. According to the World Health Organization 40 percent of the world\'s population is at risk from dengue fever. Dengue is spread by mosquitoes that breed in stagnant water and usually bite people during daytime. AFP PHOTO / Jay DIRECTO (Photo credit should read JAY DIRECTO/AFP/Getty Images) (credit:JAY DIRECTO via Getty Images)
HONDURAS-HEALTH-DENGUE(27 of34)
A boy affected by dengue fever rests at the pediatric ward of the University Hospital in Tegucigalpa on March 3, 2014. Honduran officials issued a warning light after more 1,000 people were affected by dengue during this year, and left 29 dead and 39,000 infected during past year. AFP PHOTO/Orlando SIERRA. (Photo credit should read ORLANDO SIERRA/AFP/Getty Images) (credit:ORLANDO SIERRA via Getty Images)
HONDURAS-HEALTH-DENGUE(28 of34)
A girl affected by dengue fever rests at the pediatric ward of the University Hospital in Tegucigalpa on March 3, 2014. Honduran officials issued a warning light after more 1,000 people were affected by dengue during this year, and left 29 dead and 39,000 infected during past year. AFP PHOTO/Orlando SIERRA. (Photo credit should read ORLANDO SIERRA/AFP/Getty Images) (credit:ORLANDO SIERRA via Getty Images)
MALAYSIA-HEALTH-DENGUE(29 of34)
A municipal council worker dispenses insecticide using a fogging machine during a dengue prevention spraying in Ampang, in the suburbs of Kuala Lumpur on February 11, 2014. Deaths from dengue fever have nearly tripled in Malaysia this year compared to the same period in 2013, sparking a stepped-up campaign to control the mosquitos that spread the virus. AFP PHOTO / MOHD RASFAN (Photo credit should read MOHD RASFAN/AFP/Getty Images) (credit:AFP via Getty Images)
MALAYSIA-HEALTH-DENGUE(30 of34)
A municipal council worker (R) dispenses insecticide using a fogging machine as children sit inside a car with a reflection of a Malaysian flag during a dengue prevention spraying in Ampang, in the suburbs of Kuala Lumpur on February 11, 2014. Deaths from dengue fever have nearly tripled in Malaysia this year compared to the same period in 2013, sparking a stepped-up campaign to control the mosquitos that spread the virus. AFP PHOTO / MOHD RASFAN (Photo credit should read MOHD RASFAN/AFP/Getty Images) (credit:AFP via Getty Images)
MALAYSIA-HEALTH-DENGUE(31 of34)
A Malaysian resident protects her face while engulfed in a cloud of pesticide during a dengue prevention spraying in Ampang, in the suburbs of Kuala Lumpur on February 11, 2014. Deaths from dengue fever have nearly tripled in Malaysia this year compared to the same period in 2013, sparking a stepped-up campaign to control the mosquitos that spread the virus. AFP PHOTO / MOHD RASFAN (Photo credit should read MOHD RASFAN/AFP/Getty Images) (credit:AFP via Getty Images)
PANAMA-HEALTH-DENGUE-NORIEGA-RESIDENCE-DEMOLITION(32 of34)
A police officer stands guard at the residence of Panama\'s former dictador Manuel Antonio Noriega, currently imprisoned in Panama, in Panama City, as it is being demolished on January 9, 2014 following a decision by the Health Ministry for considering it a breeding site of the Aedes mosquito that transmits dengue. Noriega, 79, who was extradited from France in 2011, is serving three 20-year prison terms for the disappearance of opposition activists during his years in power (1983-89). The former dictator has been petitioning to be placed under house arrest since 2012, arguing that his ailing health warrants it. Noriega was toppled from power in a US invasion in 1989, and spent nearly two decades in prison in the United States and France on drug charges. AFP PHOTO/ Rodrigo ARANGUA (Photo credit should read RODRIGO ARANGUA/AFP/Getty Images) (credit:RODRIGO ARANGUA via Getty Images)
PANAMA-HEALTH-DENGUE-NORIEGA-RESIDENCE-DEMOLITION(33 of34)
A police officer walks at the residence of Panama\'s former dictador Manuel Antonio Noriega, currently imprisoned in Panama, Panama City, as it is being demolished on January 9, 2014 following a decision by the Health Ministry for considering it a breeding site of the Aedes mosquito that transmits dengue. Noriega, 79, who was extradited from France in 2011, is serving three 20-year prison terms for the disappearance of opposition activists during his years in power (1983-89). The former dictator has been petitioning to be placed under house arrest since 2012, arguing that his ailing health warrants it. Noriega was toppled from power in a US invasion in 1989, and spent nearly two decades in prison in the United States and France on drug charges. AFP PHOTO/ Rodrigo ARANGUA (Photo credit should read RODRIGO ARANGUA/AFP/Getty Images) (credit:RODRIGO ARANGUA via Getty Images)
Children Deaths At Muzaffarnagar Riots Relief Camps(34 of34)
MUZAFFARNAGAR, INDIA - DECEMBER 1: Shehnaaz, 34, and husband Mohd. Shehzad, 38, show a local newspaper cutting which carried the photograph of their son who died because of dengue at Loi camp on December 1, 2013 in Muzaffarnagar, India. More than 50 children and many old age people have died because of cold and lack of medical facilities in the relief camps set up for Muzaffarnagar riots victims. (Photograph by Raj k Raj/Hindustan Times via Getty Images) (credit:Hindustan Times via Getty Images)
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